FIG. 14 illustrates an equivalent circuit of a pixel circuit of a common active matrix liquid crystal display device. Further, FIG. 15 illustrates an example of a circuit arrangement in an active matrix liquid display device of m×n pixels. As illustrated in FIG. 15, a switching element formed with a thin film transistor (TFT) is provided at each intersection of m source lines (data signal lines) and n scanning lines (scan signal lines) and, as illustrated in FIG. 14, a liquid crystal element LC and a retentive capacity Cs are connected in parallel through the TFT. The liquid crystal element LC adopts a layered structure in which a liquid crystal layer is provided between a pixel electrode and a counter electrode (common electrode). In addition, FIG. 15 briefly illustrates only a TFT and a pixel electrode (black rectangular portion) of each pixel circuit. In the retentive capacity Cs, one end is connected to the pixel electrode and the other end is connected to a capacity line LCs to stabilize a pixel data voltage held in the pixel electrode. The retentive capacity Cs provides an effect of suppressing fluctuation of the pixel data voltage held in the pixel electrode due to a leak current of a TFT, fluctuation of an electrical capacitance of the liquid crystal element LC between black display and white display due to dielectric anisotropy of liquid crystal particles, and voltage fluctuation caused by parasitic capacitance between the pixel electrode and a surrounding wiring. By sequentially controlling voltages of scanning lines, the TFTs connected to one scanning line enter a conducted state, and the pixel data voltages supplied to respective source lines are written in corresponding pixel electrodes with respect to each scanning line.
Even when display content is a still image upon normal display of full color display, by repeatedly writing the same display content in the same pixel with respect to each frame while inverting the voltage polarity applied to the liquid crystal element LC, the pixel data voltage held in the pixel electrode is updated, voltage fluctuation of pixel data is minimized, and high quality still image display is secured.
Power consumption for driving a liquid crystal display device is mostly occupied by power consumption for driving source lines by a source driver, and can be roughly expressed by a relational expression shown in following equation 1. In equation 1, P represents power consumption, f represents a refresh rate (the number of times of refresh operations in one frame per unit time), C represents a load capacitance driven by a source driver, V represents a driving voltage of the source driver, n represents the number of scanning lines and m represents the number of source lines. In addition, the refresh operation is directed to canceling fluctuation produced in a voltage (absolute value) corresponding to pixel data applied to the liquid crystal element LC by writing the pixel data again, and returning the voltage to the original voltage state corresponding to the pixel data.P∝f·C·V2·n·m  (Equation 1)
Meanwhile, when a still image is constantly displayed, display content is a still image, and therefore the pixel data voltage does not necessarily need to be updated for each frame. Hence, to further reduce the power consumption of the liquid crystal display device, a refresh frequency upon this constant display is decreased. However, when the refresh frequency is decreased, the pixel data voltage held in the pixel electrode fluctuates due to the leak current of the TFT. Further, the average potential in each frame period decreases, and therefore this voltage fluctuation causes fluctuation of display brightness (the transmittance of liquid crystal) of each pixel, and is observed as a flicker. Furthermore, there is also a concern that, for example, sufficient contract cannot be obtained, and therefore display quality decreases.
Meanwhile, for example, Patent Documents 1 and 2 disclose configurations as a method of solving a problem that display quality decreases due to a decrease in the refresh frequency upon constant display of a still image. According to the configurations disclosed in Patent Documents 1 and 2, a switching element of a pixel circuit illustrated in FIG. 14 is formed with a series circuit of two TFTs (transistors T1 and T2), an intermediate node N2 between the two TFTs is driven to have the same potential as a pixel electrode N1 using a buffer amplifier 50 of a unity gain, and a problem that display quality decreases is solved by substantially suppressing the leak current of the TFT by preventing the voltage from being applied between a source and a drain of the TFT (T2) arranged on a pixel electrode side (see FIGS. 16 and 17).
This is a solution method which takes into account a substantial increase in the leak current of the TFT following an increase in a bias voltage between the source and the drain. As illustrated in FIGS. 16 and 17, according to the configurations disclosed in Patent Documents 1 and 2, although the bias voltage between the source and the drain increases in the TFT (T1) connected to the source line SL and the leak current of the TFT is likely to increase, the leak current is compensated for by the buffer amplifier 50 and does not influence the pixel data voltage held in the pixel electrode N1. According to this configuration with the buffer amplifier 50, a problem that display quality decreases due to a decrease in the refresh frequency is solved, and power consumption is further reduced due to a decrease in the refresh frequency. Further, the configurations disclosed in Patent Documents 1 and 2 can support two or more different voltage states as the pixel data voltage held in the pixel electrode, and can realize high quality constant display having multiple tones with low power consumption.